Low density alumina balls are very useful as a catalyst or catalyst support material, for example, in the automotive catalyst application described in U.S. Pat. No. 4,051,072. For this reason, extensive work has been done to find ways to make such a product, and numerous patents have been issued. In general, two basic methods have been used to obtain low density: "burn outs" and "oil drop".
In the "burn out" approach, either activated alumina, gelatinous alumina, or microcrystalline precipitated boehmite (pseudoboehmite) is mixed with a volatile material and shaped as by pelletizing or extrusion. After aging and drying, the material is heated, causing the volatile component to vaporize or burn out, leaving a porous, low density structure. U.S. Pat. Nos. 3,856,708, 4,051,072, and 4,119,474 are examples of this approach. This method has several drawbacks, since clean burning, controlled-size additives are expensive, while the cheaper additives such as sawdust require high temperatures to burn out, tend to carbonize, and can leave undesirable ash residues. Also, these additives require precautions to prevent dust explosions during use, and in some cases to treat off-gases to prevent environmental pollution.
In the oil-drop method, soluble aluminum compounds, such as aluminum hydroxychloride, are dissolved in water and the solution dropped into an immiscible liquid where the solution is neutralized in some manner, causing the alumina to precipitate, which in turn transforms the drops into rigid spheres. Alternately, expensive microcrystalline boehmite (pseudoboehmite) is dispersed as a colloidal sol, and the sol is dropped into the immiscible fluid, causing the sol to gel either by thermal or chemical means. A number of variations of this approach are represented by U.S. Pat. Nos. 2,620,314; 3,346,336; 3,919,403; 4,116,882; 4,216,122; 4,273,735; 4,279,779; 4,301,033; 4,315,839; and 4,318,896. This method has the disadvantages of expensive raw materials, as well as chemical by-products such as ammonia, chlorides, nitrates, etc., which must be handled and eventually disposed of in some way.
In addition to low density, it is also desirable in certain applications to have a porous alumina object containing a large amount of very fine pores with diameters below about 300 .ANG.. See U.S. Pat. No. 4,001,144.